1. Field of the Invention
The present invention relates to a lens holding spindle for a grinding machine, comprising two substantially cylindrical elements mounted concentrically one inside the other and movable axially with respect to each other, the outer cylindrical element comprising at one end a recess for receiving a lens to be ground and a support for said lens, the inner cylindrical element comprising an end portion which extends axially and which comprises an endmost face engageable with one face of the lens to be ground, one of the two elements forming the body of the spindle to be fixed to the grinding machine, and clamping means mounted on one of the two cylindrical elements and operable, when actuated, to clamp the other of said two elements.
2. Description of the Prior Art
Lens holding spindles are used for example in machines for grinding spherical or toric, concave or convex surfaces on different materials such as mineral or organic glasses. Such a machine and its lens holding spindle are described for example in French Pat. No. 1 445 5122 (see also-U.S. Pat. Nos. 1,952,212, 2,879,632, 3,237,349, and 3,410,031). As is illustrated schematically in FIG. 1 of the accompanying drawings, known machines comprise essentially a lens holding spindle 1 which is movable in translation along its longitudinal axis as shown by the double arrow F.sub.1 and in rotation about a shaft 2, perpendicular to the plane of the Figure, as shown by the doubled curved arrow F.sub.2. Shaft 2 is fixed with respect to the frame of the machine and is in general vertical. The machine further comprises a rotary spindle 3 with horizontal axis, which supports at one end a grinding wheel 4 and which is movable in translation both in the direction of its longitudinal axis as shown by the double arrow F.sub.3 and in a direction perpendicular to its longitudinal axis as shown by the double arrow F.sub.4. The mechanisms for obtaining the above mentioned movements of spindle 1 and spindle 3 are not shown in FIG. 3 to the extent that they are fully described in the above mentioned French patent to which reference may be made for further details. It will simply be noted that these mechanisms comprise sliding carriages with which graduated scales forming verniers are associated, which allows the position of spindle 1 to be adjusted and set with respect to shaft 2 (dimension Z) and the position of the grinding wheel 4 with respect to shaft 2 (dimensions X and Y).
Spindle 1 comprises, in its end adjacent grinding wheel 4, a truncated cone shaped recess 5 which receives the shank 6, also in the form of a truncated cone, of a lens holder 7 which supports the lens 8 to be ground. In the case where the lens 8 is a semi-finished ophthalmic lens, i.e. a lens one 8a of the two faces 8a and 8b of which has already been ground and polished and whose other face 8b must be ground to a spherical or toric shape having the prescribed curvature or curvatures, the ophthalmic lens 8 is mounted in the lens holder 7 by means of a metal block 9, also called "molette" which is molded and bonded to the finished face 8a of the ophthalmic lens 8. For a grinding wheel 4 of a given shape and dimensions, adjustment of dimensions X and Y allows the desired toric or spherical surface to be obtained when spindle 1 and the ophthalmic lens 8 are caused to pivot about shaft 2. Adjustment of the position of spindle 1 in the direction of the double arrow F.sub. 1 (dimension Z) determines the thickness e at the center of the ophthalmic lens 8.
However, with the lens holding spindle described in French Pat. No. 1,445,552, although the dimension Z is perfectly known and may be accurately adjusted, the distance d between the apex S of the finished face 8a of lens 8 and the front face of spindle 1 is generally unknown. This distance d depends on several factors such as the curvature of the finished face 8a of lens 8, the thickness of the metal block 9, the accuracy of mounting the metal block 9 in the lens holder 7, the degree of penetration of the shank 6 of the lens holder 7 in the truncated cone shaped recess 5 of spindle 1. Some of these factors can not be measured and/or reproduced from one grinding operation to the next. This compels the operator to carry out grinding of face 8b of lens 8 in two stages. In a first stage, the operator adjusts dimension Z to a first value Z.sub.1 and effects a first grinding pass. Then, he removes the ophthalmic lens 8 from the lens holder 7 and measures the thickness at the center of lens 8. If e.sub.1 designates the thickness thus measured and e.sub.2 the desired thickness (e.sub.2 &lt;e.sub.1), the operator then adjusts Z to the value Z.sub.2 so that: EQU Z.sub.2 =Z.sub.1 -(e.sub.1 -e.sub.2)
and he then effects a second grinding pass so as to obtain the desired thickness at the center of lens 8. Such a method of operating, which requires removal of the ophthalmic lens 8 and measurement of the thickness at its center between two grinding passes, represents a waste of time which adversely effects the productivity of the machine. Furthermore, the thickness effectively obtained at the center of lens 8 after the second grinding pass may be erroneous if, before the second machining pass, lens 8 is not mounted again on spindle 1 exactly in the same axial position as the one it occupied during the first grinding pass.
The above observations are also valid in the case of the lens holding spindles described in U.S. Pat. Nos. 2,879,632, 3,237,349 and 3,410,031.
With the machine described in U.S. Pat. No. 1,952,212 the axial position of the lens holding spindle may be adjusted so as to obtain the final thickness desired at the center of the lens without intermediate measurement of this thickness during grinding. However, this result is obtained by means of a relatively complex mechanical structure, which impairs the accuracy of positioning of the lens and which is difficult to protect from the projections of water and glass particles produced during grinding. Furthermore, so as to be able to go over from grinding of a concave surface to that of a convex surface, it is necessary to reverse the relative positions of the lens and of the elements of the lens holding spindle, which complicates handling of the grinding machine. Finally, the structure described in U.S. Pat. No. 1,952,212 is difficult to adapt to a digitally controlled grinding machine.